This invention relates to a gas turbofan engine and, more particularly, to a novel arrangement for the fan section thereof.
Considerable attention has been devoted to developing gas turbine engines with the high specific thrust characteristics of a turbojet or low bypass turbofan at supersonic speeds which can also be configured to exhibit the lower specific thrust, low noise, and low fuel consumption characteristics of a higher bypass turbo-fan at subsonic speeds in order that a very efficient mixed-mission aircraft may be developed. Such engines are generally referred to as variable cycle engines.
Several design approaches for a variable cycle engine have been proposed including several modifications of mixed flow gas turbine engines. Thus, it has been proposed to vary the bypass ratio of a gas turbine engine by operating it either as a mixed flow or a separated flow turbofan by selectively mixing or separating the bypass duct stream from the core exhaust stream using diverter valves. It has also been proposed to increase the flow modulation potential of a gas turbine engine by splitting the fan into two sections, each in flow communication with a separate concentric bypass duct.
One problem associated with all such prior art variable cycle engines is the high workload imposed on the fan system to produce the desired high bypass ratios at low thrust flight. In order to supply sufficient rotational energy to produce these high fan flow rates, most prior art turbofan engines have utilized a multi-stage low pressure turbine disposed downstream of the high pressure turbine of a gas generator which operates at relatively high temperatures.